/*
 * %W% %E%
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package com.tiantian.news.util.deque;

import java.util.AbstractQueue;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
 * linked nodes.
 *
 * <p> The optional capacity bound constructor argument serves as a
 * way to prevent excessive expansion. The capacity, if unspecified,
 * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
 * dynamically created upon each insertion unless this would bring the
 * deque above capacity.
 *
 * <p>Most operations run in constant time (ignoring time spent
 * blocking).  Exceptions include {@link #remove(Object) remove},
 * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
 * #removeLastOccurrence removeLastOccurrence}, {@link #contains
 * contains}, {@link #iterator iterator.remove()}, and the bulk
 * operations, all of which run in linear time.
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.6
 * @author  Doug Lea
 * @param <E> the type of elements held in this collection
 */
public class LinkedBlockingDeque<E>
extends AbstractQueue<E>
implements BlockingDeque<E>,  java.io.Serializable {

	/*
	 * Implemented as a simple doubly-linked list protected by a
	 * single lock and using conditions to manage blocking.
	 *
	 * To implement weakly consistent iterators, it appears we need to
	 * keep all Nodes GC-reachable from a predecessor dequeued Node.
	 * That would cause two problems:
	 * - allow a rogue Iterator to cause unbounded memory retention
	 * - cause cross-generational linking of old Nodes to new Nodes if
	 *   a Node was tenured while live, which generational GCs have a
	 *   hard time dealing with, causing repeated major collections.
	 * However, only non-deleted Nodes need to be reachable from
	 * dequeued Nodes, and reachability does not necessarily have to
	 * be of the kind understood by the GC.  We use the trick of
	 * linking a Node that has just been dequeued to itself.  Such a
	 * self-link implicitly means to advance to head.
	 */

	/*
	 * We have "diamond" multiple interface/abstract class inheritance
	 * here, and that introduces ambiguities. Often we want the
	 * BlockingDeque javadoc combined with the AbstractQueue
	 * implementation, so a lot of method specs are duplicated here.
	 */

	private static final long serialVersionUID = -387911632671998426L;

	/** Doubly-linked list node class */
	static final class Node<E> {
		/**
		 * The item, or null if this node has been removed.
		 */

		E item;

		/**
		 * One of:
		 * - the real predecessor Node
		 * - this Node, meaning the predecessor is tail
		 * - null, meaning there is no predecessor
		 */

		Node<E> prev;

		/**
		 * One of:
		 * - the real successor Node
		 * - this Node, meaning the successor is head
		 * - null, meaning there is no successor
		 */

		Node<E> next;
		Node(E x, Node<E> p, Node<E> n) {
			item = x;
			prev = p;
			next = n;
		}
	}

	/** Pointer to first node */
	transient Node<E> first;
	/** Pointer to last node */
	transient Node<E> last;
	/** Number of items in the deque */
	private transient int count;
	/** Maximum number of items in the deque */
	private final int capacity;
	/** Main lock guarding all access */
	final ReentrantLock lock = new ReentrantLock();
	/** Condition for waiting takes */
	private final Condition notEmpty = lock.newCondition();
	/** Condition for waiting puts */
	private final Condition notFull = lock.newCondition();

	/**
	 * Creates a <tt>LinkedBlockingDeque</tt> with a capacity of
	 * {@link Integer#MAX_VALUE}.
	 */
	public LinkedBlockingDeque() {
		this(Integer.MAX_VALUE);
	}

	/**
	 * Creates a <tt>LinkedBlockingDeque</tt> with the given (fixed) capacity.
	 *
	 * @param capacity the capacity of this deque
	 * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
	 */
	public LinkedBlockingDeque(int capacity) {
		if (capacity <= 0) {
			throw new IllegalArgumentException();
		}
		this.capacity = capacity;
	}

	/**
	 * Creates a <tt>LinkedBlockingDeque</tt> with a capacity of
	 * {@link Integer#MAX_VALUE}, initially containing the elements of
	 * the given collection, added in traversal order of the
	 * collection's iterator.
	 *
	 * @param c the collection of elements to initially contain
	 * @throws NullPointerException if the specified collection or any
	 *         of its elements are null
	 */
	public LinkedBlockingDeque(Collection<? extends E> c) {
		this(Integer.MAX_VALUE);
		final ReentrantLock lock = this.lock;
		lock.lock(); // Never contended, but necessary for visibility
		try {
			for (E e : c) {
				if (e == null) {
					throw new NullPointerException();
				}
				if (!linkLast(e)) {
					throw new IllegalStateException("Deque full");
				}
			}
		} finally {
			lock.unlock();
		}
	}


	// Basic linking and unlinking operations, called only while holding lock

	/**
	 * Links e as first element, or returns false if full.
	 */
	private boolean linkFirst(E e) {
		// assert lock.isHeldByCurrentThread();
		if (count >= capacity) {
			return false;
		}
		Node<E> f = first;
		Node<E> x = new Node<E>(e, null, f);
		first = x;
		if (last == null) {
			last = x;
		} else {
			f.prev = x;
		}
		++count;
		notEmpty.signal();
		return true;
	}

	/**
	 * Links e as last element, or returns false if full.
	 */
	private boolean linkLast(E e) {
		// assert lock.isHeldByCurrentThread();
		if (count >= capacity) {
			return false;
		}
		Node<E> l = last;
		Node<E> x = new Node<E>(e, l, null);
		last = x;
		if (first == null) {
			first = x;
		} else {
			l.next = x;
		}
		++count;
		notEmpty.signal();
		return true;
	}

	/**
	 * Removes and returns first element, or null if empty.
	 */
	private E unlinkFirst() {
		// assert lock.isHeldByCurrentThread();
		Node<E> f = first;
		if (f == null) {
			return null;
		}
		Node<E> n = f.next;
		E item = f.item;
		f.item = null;
		f.next = f; // help GC
		first = n;
		if (n == null) {
			last = null;
		} else {
			n.prev = null;
		}
		--count;
		notFull.signal();
		return item;
	}

	/**
	 * Removes and returns last element, or null if empty.
	 */
	private E unlinkLast() {
		// assert lock.isHeldByCurrentThread();
		Node<E> l = last;
		if (l == null) {
			return null;
		}
		Node<E> p = l.prev;
		E item = l.item;
		l.item = null;
		l.prev = l; // help GC

		last = p;
		if (p == null) {
			first = null;
		} else {
			p.next = null;
		}
		--count;
		notFull.signal();
		return item;
	}

	/**
	 * Unlinks x
	 */
	void unlink(Node<E> x) {
		// assert lock.isHeldByCurrentThread();
		Node<E> p = x.prev;
		Node<E> n = x.next;
		if (p == null) {
			unlinkFirst();
		} else if (n == null) {
			unlinkLast();
		} else {
			p.next = n;
			n.prev = p;
			x.item = null;
			// Don't mess with x's links. They may still be in use by
			// an iterator.
			--count;
			notFull.signal();
		}
	}

	// BlockingDeque methods

	/**
	 * @throws IllegalStateException {@inheritDoc}
	 * @throws NullPointerException  {@inheritDoc}
	 */
	@Override
	public void addFirst(E e) {
		if (!offerFirst(e)) {
			throw new IllegalStateException("Deque full");
		}
	}

	/**
	 * @throws IllegalStateException {@inheritDoc}
	 * @throws NullPointerException  {@inheritDoc}
	 */
	@Override
	public void addLast(E e) {
		if (!offerLast(e)) {
			throw new IllegalStateException("Deque full");
		}
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 */
	@Override
	public boolean offerFirst(E e) {
		if (e == null) {
			throw new NullPointerException();
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return linkFirst(e);
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 */
	@Override
	public boolean offerLast(E e) {
		if (e == null) {
			throw new NullPointerException();
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return linkLast(e);
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 * @throws InterruptedException {@inheritDoc}
	 */
	@Override
	public void putFirst(E e) throws InterruptedException {
		if (e == null) {
			throw new NullPointerException();
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			while (!linkFirst(e)) {
				notFull.await();
			}
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 * @throws InterruptedException {@inheritDoc}
	 */
	@Override
	public void putLast(E e) throws InterruptedException {
		if (e == null) {
			throw new NullPointerException();
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			while (!linkLast(e)) {
				notFull.await();
			}
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 * @throws InterruptedException {@inheritDoc}
	 */
	@Override
	public boolean offerFirst(E e, long timeout, TimeUnit unit)
			throws InterruptedException {
		if (e == null) {
			throw new NullPointerException();
		}
		long nanos = unit.toNanos(timeout);
		final ReentrantLock lock = this.lock;
		lock.lockInterruptibly();
		try {
			while (!linkFirst(e)) {
				if (nanos <= 0) {
					return false;
				}
				nanos = notFull.awaitNanos(nanos);
			}
			return true;
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 * @throws InterruptedException {@inheritDoc}
	 */
	@Override
	public boolean offerLast(E e, long timeout, TimeUnit unit)
			throws InterruptedException {
		if (e == null) {
			throw new NullPointerException();
		}
		long nanos = unit.toNanos(timeout);
		final ReentrantLock lock = this.lock;
		lock.lockInterruptibly();
		try {
			while (!linkLast(e)) {
				if (nanos <= 0) {
					return false;
				}
				nanos = notFull.awaitNanos(nanos);
			}
			return true;
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NoSuchElementException {@inheritDoc}
	 */
	@Override
	public E removeFirst() {
		E x = pollFirst();
		if (x == null) {
			throw new NoSuchElementException();
		}
		return x;
	}

	/**
	 * @throws NoSuchElementException {@inheritDoc}
	 */
	@Override
	public E removeLast() {
		E x = pollLast();
		if (x == null) {
			throw new NoSuchElementException();
		}
		return x;
	}

	@Override
	public E pollFirst() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return unlinkFirst();
		} finally {
			lock.unlock();
		}
	}

	@Override
	public E pollLast() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return unlinkLast();
		} finally {
			lock.unlock();
		}
	}

	@Override
	public E takeFirst() throws InterruptedException {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			E x;
			while ( (x = unlinkFirst()) == null) {
				notEmpty.await();
			}
			return x;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public E takeLast() throws InterruptedException {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			E x;
			while ( (x = unlinkLast()) == null) {
				notEmpty.await();
			}
			return x;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public E pollFirst(long timeout, TimeUnit unit)
			throws InterruptedException {
		long nanos = unit.toNanos(timeout);
		final ReentrantLock lock = this.lock;
		lock.lockInterruptibly();
		try {
			E x;
			while ( (x = unlinkFirst()) == null) {
				if (nanos <= 0) {
					return null;
				}
				nanos = notEmpty.awaitNanos(nanos);
			}
			return x;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public E pollLast(long timeout, TimeUnit unit)
			throws InterruptedException {
		long nanos = unit.toNanos(timeout);
		final ReentrantLock lock = this.lock;
		lock.lockInterruptibly();
		try {
			E x;
			while ( (x = unlinkLast()) == null) {
				if (nanos <= 0) {
					return null;
				}
				nanos = notEmpty.awaitNanos(nanos);
			}
			return x;
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws NoSuchElementException {@inheritDoc}
	 */
	@Override
	public E getFirst() {
		E x = peekFirst();
		if (x == null) {
			throw new NoSuchElementException();
		}
		return x;
	}

	/**
	 * @throws NoSuchElementException {@inheritDoc}
	 */
	@Override
	public E getLast() {
		E x = peekLast();
		if (x == null) {
			throw new NoSuchElementException();
		}
		return x;
	}

	@Override
	public E peekFirst() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return (first == null) ? null : first.item;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public E peekLast() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return (last == null) ? null : last.item;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public boolean removeFirstOccurrence(Object o) {
		if (o == null) {
			return false;
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			for (Node<E> p = first; p != null; p = p.next) {
				if (o.equals(p.item)) {
					unlink(p);
					return true;
				}
			}
			return false;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public boolean removeLastOccurrence(Object o) {
		if (o == null) {
			return false;
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			for (Node<E> p = last; p != null; p = p.prev) {
				if (o.equals(p.item)) {
					unlink(p);
					return true;
				}
			}
			return false;
		} finally {
			lock.unlock();
		}
	}

	// BlockingQueue methods

	/**
	 * Inserts the specified element at the end of this deque unless it would
	 * violate capacity restrictions.  When using a capacity-restricted deque,
	 * it is generally preferable to use method {@link #offer(Object) offer}.
	 *
	 * <p>This method is equivalent to {@link #addLast}.
	 *
	 * @throws IllegalStateException if the element cannot be added at this
	 *         time due to capacity restrictions
	 * @throws NullPointerException if the specified element is null
	 */
	@Override
	public boolean add(E e) {
		addLast(e);
		return true;
	}

	/**
	 * @throws NullPointerException if the specified element is null
	 */
	@Override
	public boolean offer(E e) {
		return offerLast(e);
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 * @throws InterruptedException {@inheritDoc}
	 */
	@Override
	public void put(E e) throws InterruptedException {
		putLast(e);
	}

	/**
	 * @throws NullPointerException {@inheritDoc}
	 * @throws InterruptedException {@inheritDoc}
	 */
	@Override
	public boolean offer(E e, long timeout, TimeUnit unit)
			throws InterruptedException {
		return offerLast(e, timeout, unit);
	}

	/**
	 * Retrieves and removes the head of the queue represented by this deque.
	 * This method differs from {@link #poll poll} only in that it throws an
	 * exception if this deque is empty.
	 *
	 * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
	 *
	 * @return the head of the queue represented by this deque
	 * @throws NoSuchElementException if this deque is empty
	 */
	@Override
	public E remove() {
		return removeFirst();
	}

	@Override
	public E poll() {
		return pollFirst();
	}

	@Override
	public E take() throws InterruptedException {
		return takeFirst();
	}

	@Override
	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
		return pollFirst(timeout, unit);
	}

	/**
	 * Retrieves, but does not remove, the head of the queue represented by
	 * this deque.  This method differs from {@link #peek peek} only in that
	 * it throws an exception if this deque is empty.
	 *
	 * <p>This method is equivalent to {@link #getFirst() getFirst}.
	 *
	 * @return the head of the queue represented by this deque
	 * @throws NoSuchElementException if this deque is empty
	 */
	@Override
	public E element() {
		return getFirst();
	}

	@Override
	public E peek() {
		return peekFirst();
	}

	/**
	 * Returns the number of additional elements that this deque can ideally
	 * (in the absence of memory or resource constraints) accept without
	 * blocking. This is always equal to the initial capacity of this deque
	 * less the current <tt>size</tt> of this deque.
	 *
	 * <p>Note that you <em>cannot</em> always tell if an attempt to insert
	 * an element will succeed by inspecting <tt>remainingCapacity</tt>
	 * because it may be the case that another thread is about to
	 * insert or remove an element.
	 */
	@Override
	public int remainingCapacity() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return capacity - count;
		} finally {
			lock.unlock();
		}
	}

	/**
	 * @throws UnsupportedOperationException {@inheritDoc}
	 * @throws ClassCastException            {@inheritDoc}
	 * @throws NullPointerException          {@inheritDoc}
	 * @throws IllegalArgumentException      {@inheritDoc}
	 */
	@Override
	public int drainTo(Collection<? super E> c) {
		return drainTo(c, Integer.MAX_VALUE);
	}

	/**
	 * @throws UnsupportedOperationException {@inheritDoc}
	 * @throws ClassCastException            {@inheritDoc}
	 * @throws NullPointerException          {@inheritDoc}
	 * @throws IllegalArgumentException      {@inheritDoc}
	 */
	@Override
	public int drainTo(Collection<? super E> c, int maxElements) {
		if (c == null) {
			throw new NullPointerException();
		}
		if (c == this) {
			throw new IllegalArgumentException();
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			int n = Math.min(maxElements, count);
			for (int i = 0; i < n; i++) {
				c.add(first.item);   // In this order, in case add() throws.
				unlinkFirst();
			}
			return n;
		} finally {
			lock.unlock();
		}
	}

	// Stack methods

	/**
	 * @throws IllegalStateException {@inheritDoc}
	 * @throws NullPointerException  {@inheritDoc}
	 */
	@Override
	public void push(E e) {
		addFirst(e);
	}

	/**
	 * @throws NoSuchElementException {@inheritDoc}
	 */
	@Override
	public E pop() {
		return removeFirst();
	}

	// Collection methods

	/**
	 * Removes the first occurrence of the specified element from this deque.
	 * If the deque does not contain the element, it is unchanged.
	 * More formally, removes the first element <tt>e</tt> such that
	 * <tt>o.equals(e)</tt> (if such an element exists).
	 * Returns <tt>true</tt> if this deque contained the specified element
	 * (or equivalently, if this deque changed as a result of the call).
	 *
	 * <p>This method is equivalent to
	 * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
	 *
	 * @param o element to be removed from this deque, if present
	 * @return <tt>true</tt> if this deque changed as a result of the call
	 */
	@Override
	public boolean remove(Object o) {
		return removeFirstOccurrence(o);
	}

	/**
	 * Returns the number of elements in this deque.
	 *
	 * @return the number of elements in this deque
	 */
	@Override
	public int size() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return count;
		} finally {
			lock.unlock();
		}
	}

	/**
	 * Returns <tt>true</tt> if this deque contains the specified element.
	 * More formally, returns <tt>true</tt> if and only if this deque contains
	 * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
	 *
	 * @param o object to be checked for containment in this deque
	 * @return <tt>true</tt> if this deque contains the specified element
	 */
	@Override
	public boolean contains(Object o) {
		if (o == null) {
			return false;
		}
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			for (Node<E> p = first; p != null; p = p.next) {
				if (o.equals(p.item)) {
					return true;
				}
			}
			return false;
		} finally {
			lock.unlock();
		}
	}
	/*
	 * TODO: Add support for more efficient bulk operations.
	 *
	 * We don't want to acquire the lock for every iteration, but we
	 * also want other threads a chance to interact with the
	 * collection, especially when count is close to capacity.
	 */

//     /**
//      * Adds all of the elements in the specified collection to this
//      * queue.  Attempts to addAll of a queue to itself result in
//      * {@code IllegalArgumentException}. Further, the behavior of
//      * this operation is undefined if the specified collection is
//      * modified while the operation is in progress.
//      *
//      * @param c collection containing elements to be added to this queue
//      * @return {@code true} if this queue changed as a result of the call
//      * @throws ClassCastException            {@inheritDoc}
//      * @throws NullPointerException          {@inheritDoc}
//      * @throws IllegalArgumentException      {@inheritDoc}
//      * @throws IllegalStateException         {@inheritDoc}
//      * @see #add(Object)
//      */
//     public boolean addAll(Collection<? extends E> c) {
//         if (c == null)
//             throw new NullPointerException();
//         if (c == this)
//             throw new IllegalArgumentException();
//         final ReentrantLock lock = this.lock;
//         lock.lock();
//         try {
//             boolean modified = false;
//             for (E e : c)
//                 if (linkLast(e))
//                     modified = true;
//             return modified;
//         } finally {
//             lock.unlock();
//         }
//     }

	/**
	 * Returns an array containing all of the elements in this deque, in
	 * proper sequence (from first to last element).
	 *
	 * <p>The returned array will be "safe" in that no references to it are
	 * maintained by this deque.  (In other words, this method must allocate
	 * a new array).  The caller is thus free to modify the returned array.
	 *
	 * <p>This method acts as bridge between array-based and collection-based
	 * APIs.
	 *
	 * @return an array containing all of the elements in this deque
	 */
	// @SuppressWarnings("unchecked")
	@Override
	public Object[] toArray() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			Object[] a = new Object[count];
			int k = 0;
			for (Node<E> p = first; p != null; p = p.next) {
				a[k++] = p.item;
			}
			return a;
		} finally {
			lock.unlock();
		}
	}

	/**
	 * Returns an array containing all of the elements in this deque, in
	 * proper sequence; the runtime type of the returned array is that of
	 * the specified array.  If the deque fits in the specified array, it
	 * is returned therein.  Otherwise, a new array is allocated with the
	 * runtime type of the specified array and the size of this deque.
	 *
	 * <p>If this deque fits in the specified array with room to spare
	 * (i.e., the array has more elements than this deque), the element in
	 * the array immediately following the end of the deque is set to
	 * <tt>null</tt>.
	 *
	 * <p>Like the {@link #toArray()} method, this method acts as bridge between
	 * array-based and collection-based APIs.  Further, this method allows
	 * precise control over the runtime type of the output array, and may,
	 * under certain circumstances, be used to save allocation costs.
	 *
	 * <p>Suppose <tt>x</tt> is a deque known to contain only strings.
	 * The following code can be used to dump the deque into a newly
	 * allocated array of <tt>String</tt>:
	 *
	 * <pre>
	 *     String[] y = x.toArray(new String[0]);</pre>
	 *
	 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
	 * <tt>toArray()</tt>.
	 *
	 * @param a the array into which the elements of the deque are to
	 *          be stored, if it is big enough; otherwise, a new array of the
	 *          same runtime type is allocated for this purpose
	 * @return an array containing all of the elements in this deque
	 * @throws ArrayStoreException if the runtime type of the specified array
	 *         is not a supertype of the runtime type of every element in
	 *         this deque
	 * @throws NullPointerException if the specified array is null
	 */
	@SuppressWarnings("unchecked")
	@Override
	public <T> T[] toArray(T[] a) {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			if (a.length < count) {
				a = (T[])java.lang.reflect.Array.newInstance
						(a.getClass().getComponentType(), count);
			}

			int k = 0;
			for (Node<E> p = first; p != null; p = p.next) {
				a[k++] = (T)p.item;
			}
			if (a.length > k) {
				a[k] = null;
			}
			return a;
		} finally {
			lock.unlock();
		}
	}

	@Override
	public String toString() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return super.toString();
		} finally {
			lock.unlock();
		}
	}

	/**
	 * Atomically removes all of the elements from this deque.
	 * The deque will be empty after this call returns.
	 */
	@Override
	public void clear() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			for (Node<E> f = first; f != null; ) {
				f.item = null;
				Node<E> n = f.next;
				f.prev = null;
				f.next = null;
				f = n;
			}
			first = last = null;
			count = 0;
			notFull.signalAll();
		} finally {
			lock.unlock();
		}
	}

	/**
	 * Returns an iterator over the elements in this deque in proper sequence.
	 * The elements will be returned in order from first (head) to last (tail).
	 * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
	 * will never throw {@link ConcurrentModificationException},
	 * and guarantees to traverse elements as they existed upon
	 * construction of the iterator, and may (but is not guaranteed to)
	 * reflect any modifications subsequent to construction.
	 *
	 * @return an iterator over the elements in this deque in proper sequence
	 */
	@Override
	public Iterator<E> iterator() {
		return new Itr();
	}

	/**
	 * Returns an iterator over the elements in this deque in reverse
	 * sequential order.  The elements will be returned in order from
	 * last (tail) to first (head).
	 * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
	 * will never throw {@link ConcurrentModificationException},
	 * and guarantees to traverse elements as they existed upon
	 * construction of the iterator, and may (but is not guaranteed to)
	 * reflect any modifications subsequent to construction.
	 */
	@Override
	public Iterator<E> descendingIterator() {
		return new DescendingItr();
	}

	/**
	 * Base class for Iterators for LinkedBlockingDeque
	 */
	private abstract class AbstractItr implements Iterator<E> {
		/**
		 * The next node to return in next()
		 */
		Node<E> next;

		/**
		 * nextItem holds on to item fields because once we claim that
		 * an element exists in hasNext(), we must return item read
		 * under lock (in advance()) even if it was in the process of
		 * being removed when hasNext() was called.
		 */
		E nextItem;

		/**
		 * Node returned by most recent call to next. Needed by remove.
		 * Reset to null if this element is deleted by a call to remove.
		 */
		private Node<E> lastRet;

		abstract Node<E> firstNode();
		abstract Node<E> nextNode(Node<E> n);
		AbstractItr() {
			// set to initial position
			final ReentrantLock lock = LinkedBlockingDeque.this.lock;
			lock.lock();
			try {
				next = firstNode();
				nextItem = (next == null) ? null : next.item;
			} finally {
				lock.unlock();
			}
		}

		/**
		 * Advances next.
		 */

		void advance() {
			final ReentrantLock lock = LinkedBlockingDeque.this.lock;
			lock.lock();
			try {
				// assert next != null;
				Node<E> s = nextNode(next);
				if (s == next) {
					next = firstNode();
				} else {
					// Skip over removed nodes.
					// May be necessary if multiple interior Nodes are removed.
					while (s != null && s.item == null) {
						s = nextNode(s);
					}
					next = s;
				}
				nextItem = (next == null) ? null : next.item;
			} finally {
				lock.unlock();
			}
		}

		@Override
		public boolean hasNext() {
			return next != null;
		}

		@Override
		public E next() {
			if (next == null) {
				throw new NoSuchElementException();
			}
			lastRet = next;
			E x = nextItem;
			advance();
			return x;
		}

		@Override
		public void remove() {
			Node<E> n = lastRet;
			if (n == null) {
				throw new IllegalStateException();
			}
			lastRet = null;

			final ReentrantLock lock = LinkedBlockingDeque.this.lock;
			lock.lock();
			try {
				if (n.item != null) {
					unlink(n);
				}
			} finally {
				lock.unlock();
			}
		}
	}

	/** Forward iterator */
	private class Itr extends AbstractItr {
		@Override
		Node<E> firstNode() { return first; }
		@Override
		Node<E> nextNode(Node<E> n) { return n.next; }
	}


	/** Descending iterator */
	private class DescendingItr extends AbstractItr {
		@Override
		Node<E> firstNode() { return last; }
		@Override
		Node<E> nextNode(Node<E> n) { return n.prev; }
	}

	/**
	 * Save the state of this deque to a stream (that is, serialize it).
	 *
	 * @serialData The capacity (int), followed by elements (each an
	 * <tt>Object</tt>) in the proper order, followed by a null
	 * @param s the stream
	 */
	private void writeObject(java.io.ObjectOutputStream s)
			throws java.io.IOException {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			// Write out capacity and any hidden stuff
			s.defaultWriteObject();
			// Write out all elements in the proper order.
			for (Node<E> p = first; p != null; p = p.next) {
				s.writeObject(p.item);
			}
			// Use trailing null as sentinel
			s.writeObject(null);
		} finally {
			lock.unlock();
		}
	}

	/**
	 * Reconstitute this deque from a stream (that is,
	 * deserialize it).
	 * @param s the stream
	 */
	private void readObject(java.io.ObjectInputStream s)
			throws java.io.IOException, ClassNotFoundException {
		s.defaultReadObject();
		count = 0;
		first = null;
		last = null;
		// Read in all elements and place in queue
		for (;;) {
			@SuppressWarnings("unchecked")
			E item = (E)s.readObject();
			if (item == null) {
				break;
			}
			add(item);
		}
	}

}
